Replace a Regex capture group with uppercase in Javascript

Question

I'd like to know how to replace a capture group with its uppercase in JavaScript. Here's a simplified version of what I've tried so far that's not working:

> a="foobar"
'foobar'
> a.replace( /(f)/, "$1".toUpperCase() )
'foobar'
> a.replace( /(f)/, String.prototype.toUpperCase.apply("$1") )
'foobar'

Would you explain what's wrong with this code?

Answer 1

You can pass a function to replace.

var r = a.replace(/(f)/, function(v) { return v.toUpperCase(); });

Explanation

a.replace( /(f)/, "$1".toUpperCase())

In this example you pass a string to the replace function. Since you are using the special replace syntax ($N grabs the Nth capture) you are simply giving the same value. The toUpperCase is actually deceiving because you are only making the replace string upper case (Which is somewhat pointless because the $ and one 1 characters have no upper case so the return value will still be "$1").

a.replace( /(f)/, String.prototype.toUpperCase.apply("$1"))

Believe it or not the semantics of this expression are exactly the same.